1. Field of the Invention
The present invention relates to an upconverter for a wireless magnetic resonance imaging (MRI) scanner, in particular where the orientation of the local coil antenna is unpredictable, or its position cannot be guaranteed to be suitable for reception or transmission.
2. Description of the Prior Art and Related Subject Matter
MRI scanners use a combination of a strong constant magnetic field (B0) from a superconducting magnet which is modified by gradient fields generated by gradient coils, together with a rotating magnetic field (B1) from a radio frequency (RF) antenna to excite nuclear magnetic resonances in the body that generate short term RF signals that are received to build up a tomographic image.
All current-generation MRI scanners employ arrays of local coils mounted in close proximity to the scanned patient to receive the RF with maximum possible signal to noise ratio (SNR). The local coils that receive signals from the back of the patient are mounted in the patient table. Local coils that receive signals from the front of the patient are arranged into ‘mats’ that are carefully placed over the patient. Associated with each mat is a flexible cable typically containing one co-axial line for each local coil. The cables interact with the B1 field and with the signals generated from the patient so ‘traps’ (high impedance sections) must be included at regular (typically λ/8) intervals. These add cost and inconvenience to the structure. In use, the requirement to connect the cables and sterilize them between scanning one patient and the next leads to increased down-time between scans. It is therefore desirable that the cables be eliminated.
Co-pending patent applications 2009P05901GB (GB0905752.2) (corresponding to U.S. application with Attorney Docket No. P10,0068 filed simultaneously herewith) and 2009P05909 GB (GB0905755.5) (corresponding to U.S. application with Attorney Docket No. P10,0070, filed simultaneously herewith) describe a wireless coils implementation that makes use of parametric amplifiers, each connected to one of a number of local coils and each also connected to a microwave antenna, hereinafter referred to as a patient microwave antenna. Co-pending application 2009P05902 GB (GB0905753.0) (corresponding to U.S. application with Attorney Docket No. P10,0069 filed simultaneously herewith) describes an alternative structure based on a two port mixer and low noise amplifier. In addition, whether parametric amplifiers or mixers are used as upconverter cores, the bore of the scanner is lined with microwave antennas, hereinafter referred to as bore microwave antennas, tuned to the same frequency as the other antennas. It is arranged that the bore microwave antennas transmit a local oscillator signal that is received by the patient microwave antennas. This signal provides the necessary power and means to make the parametric amplifiers, mixers and low noise amplifiers operational to upconvert the local signal to the microwave frequencies. The upconverted signals are radiated from the patient microwave antenna and received at the bore microwave antennas. Receivers, connected to one or more of the bore microwave antennas use the same local oscillator to downconvert the signals back to the original frequency. The patient and bore microwave antennas use linear polarization such as may be provided, for example, by the use of dipole or slot antennas.
This concept works well where the local coils are mounted on a mat that is laid across the patient, hereinafter referred to as a patient mat, because the polarization of the patient and bore microwave antennas can readily be aligned. However, in an MRI scanner with wireless coils, using a two port upconverter core, there are two mechanisms which cause a reduction in desired performance. The first is the local oscillator illumination at the patient microwave antenna not being adequate for powering the upconverter core; and the second is that the view of the patient antenna by the bore antenna connected to the receiver is not good enough (e.g. the bore antenna out of range, obscured, or not in the main beam), even if the local oscillator (LO) illumination provides adequate power for the upconverter core.